Television cameras produce an optical image of an object on an imaging area of a solid-state imaging device. The imaging area is typically defined by an array comprising rows and columns of photo-elements. The camera scans the charges on the photo-elements at a scanning frequency. The camera converts the charges to an electrical signal representing an image of the object. The scanning frequency corresponds to the television system employed. For example., in the NTSC system a scanning frequency of 60 fields/sec is employed.
Solid-state devices employed in imaging devices include Charge Coupled Devices (CCD) and Complementary Metal Oxide Semiconductor (CMOS) devices. Recently active pixel sensors (APS) comprising CMOS transistors have been employed. An active pixel sensor (APS) is an image sensor comprising an integrated circuit including an array of pixels. Each pixel comprises a photo-detector as well as three or more transistors. Since it can be produced by an ordinary CMOS manufacturing process APS is emerging as an inexpensive alternative to CCDs.
The resolution of images obtained with conventional solid state imaging devices is limited by the rate at which the camera can scan the imaging area. The more pixels in the imaging area the more time is needed to “readout” the array. For moving images imager readout rates limit the number of pixels comprising displayed images. At the same time larger screens demand more pixels to adequately represent imaged objects. There is a need for high resolution imagers capable of representing objects with a large number of imaging elements (pixels) without increasing the time required to readout the charges from the array of imaging elements.